Field of the Invention
This invention relates to test instruments and, in particular, to a method of magnetographic inspection of the quality of materials and a device therefor. It is designed to provide magnetic recording of flaw fields in the process of magnetographic flaw detection, for example, during quality testing of ferromagnetic products, welded joints or quality inspection of their parts.
Description of the Prior Art
Known in the art are quality test magnetographic methods and devices realizing these methods (cf., for example, U.S. Pat. No. 3,341,771) wherein a magnetizing medium is applied on a test material, and they are both magnetized. To this end, a magnetizing coil installed in the central portion of a magnetizing yoke is connected to a magnetizing current source, and flaw leakage fields are recorded on a magnetic-recording medium which is in contact with the surface of the material. The information contained in the recorded magnetogram is the evidence of the quality of the tested material and the exact location of flaws.
But this prior art method is deficient in that the inspection is not reliable because the leakage fields of flaws are recorded against the background of interferences caused by irregularities of the surface of the tested material. The level of such interference often exceeds the useful signal magnitude. In addition, this testing method cannot provide the desired intimate contact between the magnetic-recording medium and the surface of the tested material, which adds to the inaccurateness of the inspection data.
Also known in the art is a method for magnetographic flaw detection in ferromagnetic products (cf., for example, FRG Pat. No. 2,253,688, IPC G 01 N), wherein a temporary self-sustaining contact is achieved between the magnetic tape and the surface of the test zone on a product. To this end, adhesion between one side of the magnetic tape and said surface is attained by means of a self-adhesive magnetic tape having one or two additional layers of adhesive and suitable for subsequent electronic read-out. In this case, an intimate contact of the magnetic tape medium and the test zone on the product surface is ensured.
But this method is not free of disadvantages which consist in lower efficiency of testing and inconveniences due to additional technological operations involving application of talcum powder onto the additional adhesive layer or selective dissolution of the adhesive coating, which is necessary to neutralize the adhesive coating after the recording is completed and before the read-out of this recording in an electronic reader.
One more disadvantage of this method consists in that the technique of obtaining information on the product quality becomes more complicated. It involves application of additional, e.g. PVC, films to protect accidental adhesion of the magnetic tape. Such tape is inconvenient in application from a roll equipped with an unrolling device. In addition, when the surface of the tested material is irregular and has sharp angles and thicknesses, the level of interferences is still often higher than the level of the useful signal, which is a serious deficiency of this method.
Also known in the art is a method of magnetographic inspection of welded joints (cf., USSR Inventor's Certificate No. 564,583, IPC G 01 N 27/82, published in "Biulleten Izobreteny" No. 25, 1975), wherein the tested product and a magnetic-recording medium applied over the welded seam are magnetized together by static and alternating magnetic fields. The tested member and the tape are magnetized in succession by the static and alternating magnetic fields whose directions are opposite.
Also known in the art is a magnetizing device to realize this method of magnetographic inspection (cf., for example, USSR Inventor's Certificate No. 315,112, IPC G 01 N 27/82, 1969), which comprises a magnetic core and a magnetizing coil placed in the central portion of said core, the coil being connected to a magnetizing current source, and a clamping plate on which a recording medium is placed. A two-position holding lock is provided for the clamping plate carrying the magnetic recording medium in this magnetizing device in order to improve the sensitivity of the magnetographic inspection and to reduce the interferences caused by the reinforcement bead of the welded seam.
The prior art methods and devices do not provide a uniform magnetic flux in the test zone if the test object has bulges and surface irregularities. They also offer no means for assessment and compensation of the effect of air gaps due to loose and dissimilar mating of the poles of the magnetizing device to the object, which is often the case in practical applications. Unstable magnetic contact is also often the reason of inaccurate assessment of the quality of the tested material.